1. Field Of The Invention
The present invention relates to a method of stimulating coal seams to improve the production of gas from the coal reservoir.
2. Prior Art
Many subterranean coal reservoirs have large volumes of both water and hydrocarbon gases, primarily methane, trapped therein. The gas found in coal is believed to have originated from the coal during its formation. The gas in the coal has been found to be adsorbed onto the internal area surface of the coal.
Presently, methane is produced from coal reservoirs through wells which are drilled into the coal seam from the surface. Once a well has been drilled and completed, the coal seam is often hydraulically fractured in much the same way as are the more conventional gas bearing formations such as sandstone or the like. Fracturing of the coal seam is believed to aid in bypassing well bore damage, distributing the pressure at or near the wellbore and accelerating the dewatering and pressure drawdown in the coal seam. The results from actual fracturing operations in coal seams indicate that the fracture mechanics and fluid flow behavior in the cleated coal formations are substantially different from those which are believed to occur in the more conventional sandstone or like formations. For a discussion of the differences and problems encountered in fracturing coal seams see Palmer et al, Paper No. 8993 entitled "Analyses Of Unconventional Behavior Observed During Coalbed Fracturing Treatments", Proceedings of the 1989 Coalbed Methane Symposium, The University Of Alabama, Tuscaloosa, April 1989 and R. G. Jeffrey et al, Paper No. 8992 entitled "Hydraulic Fracturing To Enhance Production Of Methane From Coal Seams", Proceedings Of The 1989 Coalbed Methane Symposium, The University Of Alabama, Tuscaloosa, the disclosures of which are incorporated herein.
Usually, the coalbed methane production process begins by drilling at least one wellbore into the coal seam. The wellbore may initially produce some water or a small amount of gas from the coal matrix. Sustained production is achieved by performing a fracturing operation upon the wellbore. In a typical fracturing process, a pad fluid first is pumped down the wellbore and into the coal-containing formation to initiate and propagate fractures in the formation. The pad fluid is followed with a proppant-ladened slurry that is introduced through the wellbore into the created fractures. The slurry forces the pad further into the created fractures thereby extending the fractures while introducing proppants into the crested fracture to maintain the fracture in an open condition at the conclusion of the treatment.
Even though hydraulic fracturing has been widely used to stimulate the production of methane from coal seams, it has often not produced the degree of stimulation expected or desired. For a discussion of various techniques that have been employed see Palmer et al, Paper No. 9139 entitled "Comparison Between Gel-Fracture and Water-Fracture Stimulations In The Black Warrior Basin", Puri et al, Paper No. 9169 entitled "Damage To Coal Permeability During Hydraulic Fracturing" and Sparks et al, Paper No. 9159 entitled "A Comparison Of Completion Techniques in the Cedar Cove Field, Black Warrior Basin, Alabama", all presented at the Proceedings of the 1991 Coalbed Methane Symposium, The University of Alabama, Tuscaloosa, May 1991, the disclosures of which are incorporated herein by reference.